Power amplifier

ABSTRACT

A power amplifier is disclosed which reliably amplifies an input signal and decreases power consumption. A pre-driving circuit pre-drives a driving circuit of a power amplifying circuit in response to an input signal. The driving circuit drives an output circuit in the power amplifying circuit. A detecting circuit detects the level of the output of the driving circuit. The pre-driving circuit receives the output of the detecting circuit and supplies the pre-driving circuit with a pre-driving current varying in response to the output of the detecting circuit. Consequently, the pre-driving current can be designed to change in response to demand, i.e., the pre-driving current can be low when demand is low and increase as the occasion demands.

BACKGROUND OF THE INVENTION

This invention relates to a power amplifier, particularly a power amplifier for use in various audio circuits.

In an audio output stage of an audio apparatus, such as used in a television receiver, a power amplifying circuit amplifies an audio signal so that the audio signal can drive a loudspeaker. Usually, from the viewpoint of power efficiency, the power amplifying circuit employs a class B power amplifying circuit which comprises a push-pull transistor output circuit and a driving circuit driving the output circuit. This type of power amplifying circuit normally is pre-driven by a pre-driving circuit in response to an audio input signal; the pre-driving circuit usually comprises a class A amplifying circuit. Furthermore, the pre-driving circuit supplies a pre-driving current to the driving circuit of the power amplifying circuit. In the prior art, the pre-driving circuit generates a constant pre-driving current corresponding to the maximum pre-driving current for the driving circuit irrespective of the level of the incoming audio signal. Accordingly, the pre-driving circuit wastefully consumes power.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a power amplifier which reliably amplifies an input signal and decreases power consumption. Another object of the present invention is to provide a power amplifier including a circuit which prevents the power amplifier from being destroyed by excessive current.

According to the present invention, a pre-driving circuit pre-drives a driving circuit of a power amplifying circuit in response to an input signal. The driving circuit drives an output circuit in the same power amplifying circuit. A detecting circuit detects the level of an output of the driving circuit. The pre-driving circuit is responsive to the output of the detecting circuit to supply the pre-driving circuit with a pre-driving current varying in response to the output of the detecting circuit. As a result, the pre-driving current can be designed to change in response to demand, i.e., the current can be low when demand is low and increase as demand increases.

The objects and advantages of the present invention will become apparent to persons skilled in the art from a study of the following description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a power amplifier according to the present invention.

FIG. 2 is a waveform diagram illustrating waveform signals of various points of FIG. 1.

FIG. 3 is a circuit diagram of another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A quasi-complementary single ended push-pull (SEPP) power amplifier according to the present invention is shown in FIG. 1. An audio input signal S_(i) is supplied to the base electrode of a transistor Q₁₁ via a condenser C₁₁. The base electrode of transistor Q₁₁ is connected to a ground line 100 through a resistor R₁₄, while ground line 100 is connected to the negative electrode of a reference voltage source E₁₁. The base electrode of transistor Q₁₁ is also connected to a power supply line 200 through resistors R₁₃ and R₂₆, while power supply line 200 is connected to the positive electrode of source E₁₁. The connection point of resistors R₁₃ and R₂₆ is grounded through a condenser C₁₂. The emitter electrode of transistor Q₁₁ is connected to the connection point of resistors R₁₃ and R₂₆ through a constant current source I₁₁. The constant current source I₁₁ is also connected to the emitter electrode of transistor Q₁₂. The collector electrode of transistor Q₁₁ is connected to the collector electrode of transistor Q₁₃, while the emitter electrode of transistor Q₁₃ is connected to ground line 100 through a resistor R₁₁. The collector electrode of transistor Q₁₂ is connected to the collector electrode of a transistor Q₁₄, while the base electrode of transistor Q₁₄ is connected to its collector electrode and the base electrode of transistor Q₁₃. The emitter electrode of transistor Q₁₄ is connected to ground line 100 through a resistor R₁₂. Transistors Q₁₁ and Q₁₂ constitute a differential amplifier 11 which is a primary stage of this power amplifier. Transistors Q₁₃ and Q₁₄ constitute a current mirror circuit.

An amplified audio signal is supplied from the collector electrode of transistor Q₁₁ to the base electrode of a transistor Q₁₅. The emitter electrode of transistor Q₁₅ is connected to the base electrode of a transistor Q₁₆ and to ground line 100 through a resistor R₁₅, while its collector electrode is connected to the collector electrode of transistor Q₁₆. The emitter electrode of transistor Q₁₆ is connected to ground line 100. Transistors Q₁₅ and Q₁₆ are arranged in a Darlington type connection as a class A amplifier and constitute one portion 12 of a pre-driving circuit.

The audio signal output voltage is supplied from the collector electrode of transistor Q₁₆ to the base electrode of a transistor Q₂₁ and to the base electrode of a transistor Q₁₉ through diodes D₁₁ and D₁₂. The collector electrode of transistor Q₁₉ is connected to power supply line 200 through a resistor R₃₀, while its emitter electrode is connected to the base electrode of a transistor Q₁₇. The collector electrode of transistor Q₂₁ is connected to ground line 100 through a resistor R₂₉ and to the base electrode of a transistor Q₁₈, while its emitter electrode is connected to the emitter electrode of a transistor Q₂₀. The base electrode of transistor Q₂₀ is connected to the emitter electrode of transistor Q₁₇ through diodes D₁₃ and D₁₄, while its collector electrode is connected to power supply line 200.

The collector electrode of transistor Q₁₇ is connected to power supply line 200, while its emitter electrode is connected to an ouput line L₁₁. The emitter electrode of transistor Q₁₈ is connected to ground line 100, while its collector electrode is connected to output line L₁₁. Output line L₁₁ is connected to a loudspeaker SP through a condenser C₁₃, and to power supply line 200 through a condenser C₁₄ and resistor R₃₀. Further, output line L₁₁ is connected to the base electrode of transistor Q₁₂ through a feedback resistor R₂₇. The base electrode of transistor Q₁₂ is also connected to ground line 100 through a condenser C₁₅ and a resistor R₂₈.

Transistors Q₁₇ and Q₂₁ constitute a power amplifying circuit 13 which is the last stage of this power amplifier. In power amplifying circuit 13, transistors Q₁₇ and Q₁₈ constitute output transistors which perform a push-pull operation, and transistors Q₁₉, Q₂₀ and Q₂₁ are the driving transistors for transistors Q₁₇ and Q₁₈.

The collector electrode of a transistor Q₂₂ is connected to the base electrode of transistor Q₁₉, while its emitter electrode is connected to power supply line 200 through a resistor R₁₆ and resistor R₃₀. The base electrode of transistor Q₂₂ is grounded through a resistor R₁₈. The collector electrode of a transistor Q₂₃ is connected to the base electrode of transistor Q₂₀, while its emitter electrode is connected to power supply line 200 through a resistor R₁₇ and resistor R₃₀ ; its base electrode is connected to the base electrode of transistor Q₂₂. The base and collector electrodes of a transistor Q₂₄ are connected to the base electrode of transistor Q₂₂, while its emitter electrode is connected to power supply line 200 through a resistor R₁₉ and resistor R₃₀. Transistors Q₂₂ to Q₂₄ constitute the other portion 12' of the pre-driving circuit which supplies a pre-driving current I_(C) to transistors Q₁₉ to Q₂₁ in power amplifying circuit 13.

The base electrode of a transistor Q₂₅ is connected to the base electrode of transistor Q₁₇ through a resistor R₂₀ and to output line L₁₁ through a resistor R₂₁. The emitter electrode of transistor Q₂₅ is connected to output line L₁₁ through a resistor R₂₂, while its collector electrode is connected to the emitter electrode of transistor Q₂₄. The base electrode of a transistor Q₂₆ is connected to the base electrode of transistor Q₁₈ through a resistor R₂₃ and to ground line 100 through a resistor R₂₄. The emitter electrode of transistor Q₂₆ is connected to ground line 100 through a resistor R₂₅, while its collector electrode is connected to the emitter electrode of transistor Q₂₄. Transistors Q₂₅ and Q₂₆ constitute a detecting circuit 14 which detects the level of signals supplied to the base electrodes of transistors Q₁₇ and Q₁₈, and generates currents I_(A) and I_(B) in response to the detected signal level.

Currents I_(A) and I_(B) from detecting circuit 14 are supplied to the emitter electrode of transistor Q₂₄ in the other portion 12' of the driving circuit. As detailed below, transistors Q₂₂ and Q₂₃ supply transistors Q₁₉, Q₂₀ and Q₂₉ with the pre-driving current I_(C), which increases in response to an increase in the audio signal supplied to transistors Q₁₇ and Q₁₈. To the contrary, decreases in the audio signal result in a decrease of the pre-driving current I_(C). Thus, since the other portion 12' of the pre-driving circuit generates the pre-driving current I_(C) in response to the level of the audio signal supplied to power amplifying circuit 13, the pre-driving current I_(C) can be designed to be small to thereby largely reduce power consumption. On the contrary, in power amplifiers of the prior art, the pre-driving circuit generates a constant pre-driving current corresponding to the maximum pre-driving current required at the maximum level of the audio signal. Therefore, such prior art pre-driving circuits wastefully consume power.

A diode D₁₅ is connected in parallel with resistor R₁₉ in the other portion 12' of the pre-driving circuit. Diode D₁₅ constitutes a restricting circuit 15 which prevents the pre-driving current I_(C) generated by transistors Q₂₂ and Q₂₃ from increasing over the predetermined value. Therefore, transistors Q₁₇ to Q₂₁ are not destroyed.

Next, the operation of the power amplifier is described with reference to the waveforms of FIG. 2. After the audio input signal is amplified by differential amplifier 11 and the one portion 12 of the pre-driving circuit, its voltage waveform is supplied to power amplifying circuit 13. During a positive half cycle of the audio signal, the pre-driving current I_(C) from transistor Q₂₂ flows into the base electrode of transistor Q₁₉, and then transistor Q₁₉ conducts. Transistor Q₁₇ is driven by transistor Q₁₉ and a current, which is shown in FIG. 2(a), flows at the collector electrode of transistor Q₁₇. At this time, transistors Q₂₀, Q₂₁ and Q₁₈ are cut off. During a negative half cycle of the audio signal, transistors Q₂₀ and Q₂₁ conduct to drive transistors Q₁₈, having the collector current flow shown in FIG. 2b. At this time, the pre-driving current I_(C) from transistor Q₂₂ flows into diodes D₁₁ and D₁₂, and transistors Q₁₉ and Q.sub. 17 are cut off. Thus, transistors Q₁₇ and Q₁₈ perform a push-pull operation to supply loudspeaker SP with an amplified audio output signal which has the waveform shown in FIG. 2(c).

In the current wave forms of FIGS. 2(a) and (b), the flat portion is the collector current of each of transistors Q₁₇, Q₁₈ (i.e., an idle current idle) which is required when the audio signal does not exist.

As shown in FIG. 2(d), the detected current I_(A) flows at the collector electrode of transistor Q₂₅ of detecting circuit 14 in response to the potential between the base and emitter electrodes of transistor Q₁₇ (i.e., the positive level of the audio signal). When the potential between the base and emitter electrodes of transistor Q₁₇ increases, the detected current I_(A) increases. When the potential between the base and emitter electrodes of transistor Q₁₈ decreases, the detected current I_(A) decreases. As shown in FIG. 2(e), a detected current I_(B) likewise flows at the collector electrode of transistor Q₂₆ in response to the potential between the base and emitter electrodes of transistor Q₁₈ (i.e., the negative level of the audio signal).

Both detected currents I_(A) and I_(B) also flow through resistor R₁₉ in the other portion 12' of pre-driving circuit. When detected currents I_(A) and I_(B) increase, the voltage drop across resistor R₁₉ increases, and the base potential of transistors Q₂₂ and Q₂₃ decreases. Therefore, the collector current of transistors Q₂₂ and Q₂₃ (i.e., the pre-driving current I_(C)) increases, as shown in FIG. 2(f). Consequently, the driving current supplied from transistors Q₁₉, Q₂₀ and Q₂₁ increases and the voltage between the base and emitter electrodes of transistors Q₁₇ and Q₁₈ increases. On the contrary, when the detected currents I_(A) and I_(B) decrease, the votage drop across resistor R₁₉ decreases, and the base potential of transistors Q₂₂ and Q₂₃ increases. Therefore, the collector current I_(C) of transistors Q₂₂ and Q₂₃ decreases, as shown in FIG. 2(f). Consequently, the driving current supplied from transistors Q₁₉, Q₂₀ and Q₂₁ decreases and the voltage between the base and emitter electrodes of transistors Q₁₇ and Q₁₈ decreases. Thus, when the voltage between the base and emitter electrodes of transistors Q₁₇ and Q₁₈ is high, namely, the level of the audio signal is large, the pre-driving current I_(C) from transistors Q₂₂ and Q₂₃ is large and the gain of transistors Q₁₇ and Q₁₈ is large. When the voltage between the base and emitter electrodes of transistors Q₁₇ and Q₁₈ is low, namely, the level of the audio signal is small or the audio signal does not exist, the pre-driving current I_(C) from transistors Q₂₂ and Q₂₃ is small and the gain of transistors is small.

As a result, the pre-driving current I_(C) changes in response to demand; this current need not be constant and large. Accordingly, the pre-driving circuits 12 and 12' can reduce the power consumption when the level of the audio signal is small or the audio signal does not exist. Further, the power amplifier can reliably amplify the audio input signal and supply it to loudspeaker SP. On the other hand, in the power amplifiers of the prior art, the pre-driving circuit must generate a constant pre-driving current shown by a broken line in FIG. 2(f). Accordingly, prior art pre-driving circuits wastefully consume power.

Next, the function of restricting circuit 15 is described in detail. When the voltage drop across resistor R₁₉ is larger than the forward voltage of diode D₁₅ in response to the increase of detected currents I_(A) and I_(B), diode D₁₅ conducts and the detected currents flow through resistor R₁₉ and diode D₁₅. The voltage drop across resistor R₁₉ is fixed at the forward voltage of diode D₁₅. Accordingly, the base potential of transistors Q₂₂ and Q₂₃ cannot drop under the predetermined value and the pre-driving current I_(C) likewise cannot increase. As a result, transistors Q₁₇ to Q₂₁ are not destroyed. Restricting circuit 15 effectively functions when the level of the audio signal is very large and a load (i.e., loudspeaker SP) shorts.

Further, resistors R₂₂ and R₂₅, which are connected to the emitter electrodes of transistors Q₂₅ and Q₂₆ as well as restricting circuit 15, perform a restricting function. Resistors R₂₂ and R₂₅ provide positive feedback and prevent the extreme current from flowing into the base electrodes of transistors Q₂₅ and Q₂₆. As a result, the pre-driving current I_(C) does not increase excessively, and the transistors Q₁₇ to Q₂₁ are not destroyed

A modification of the above circuit is possible. Transistor Q₂₄ can be removed and two diodes can be connected in parallel with resistor R₁₉ in place of diode D₁₅. The circuit, as modified, would still operate as described above.

Another embodiment of the present invention is shown in FIG. 3. For the sake of simplicity, like reference numerals are used to designate like or equivalent portions for the circuit of FIG. 1, and the explanation of these portions is omitted. After an audio input signal Si is amplified by differential amplifier 11, which is equivalent to the differential amplifier 11 in FIG. 1, it is supplied to the base electrode of a transistor Q₃₄. The transistor Q₃₄ constitutes one portion 12 of a pre-driving circuit. Its collector electrode is connected to a ground line 100, while its emitter electrode is connected to the base electrode of a transistor Q₁₉ through a diode D₁₆ and to the base electrode of a transistor Q₂₁. Its emitter electrode is also connected to its base electrode through a condenser C₁₆.

The voltage waveform of the audio signal from transistor Q₃₄ is supplied to transistors Q₁₉, Q₂₀ and Q₂₁. Transistors Q₁₉, Q₂₀ and Q₂₁ drive output transistors Q₁₇ and Q₁₈. Transistors Q₁₇ to Q₂₁, like transistors Q₁₇ to Q₂₁ in FIG. 1, constitute a power amplifying circuit 13. Power amplifying circuit 13 has substantially the same constitution as power amplifying circuit 13 in FIG. 1 except that the base electrode of transistor Q₂₀ is connected to the emitter electrode of transistor Q₁₇ through diodes D₁₇, D₁₈ and D₁₉, and the collector electrode of transistor Q₁₇ is connected to a power supply line 200 through resistors R₃₀ and R₃₃.

A detecting circuit 14, which is connected to the base electrodes of transistors Q₁₇ and Q₁₈, has the same constitution and function as detecting circuit 14 in FIG. 1. The collector electrodes of transistors Q₂₅ and Q₂₆ of detecting circuit 14 are connected to the emitter electrode of a transistor Q₃₂.

The emitter electrode of transistor Q₃₂ is also connected to power supply line 200 through resistors R₃₃ and R₃₀. Its collector electrode is connected to ground line 100 through a constant current source I₁₂, while its base electrode is connected to the emitter electrode of a transistor Q₃₃ through a resistor R₃₄. The collector electrode of transistor Q₃₃ is connected to ground line 100, while its base electrode is connected to the collector electrode of transistor Q₃₂. The base electrodes of transistors Q₂₂ and Q₂₃ are connected to the base electrode of transistors Q₃₂. The emitter electrode of transistor Q₂₂ is connected to power supply line 200 through resistors R₃₄ and R₃₀, while its collector electrode is connected to the base electrode of transistor Q₁₉. The emitter electrode of transistor Q₂₃ is connected to power supply line 200 through resistors R₃₅ and R₃₀, while its collector electrode is connected to the base electrode of transistor Q₂₀. Transistors Q₂₂, Q₂₃ and Q₃₂ constitute the other portion 12' of the pre-driving circuit and supply a pre-driving current I_(C) to power amplifying circuit 13.

A diode D₁₅ is connected in parallel with resistor R₃₃ and constitutes a restricting circuit 15. Further, a second restricting circuit 16 is employed as described below. The collector electrode of a transistor Q₃₁ is connected to the base electrode of transistor Q₁₉, while its emitter electrode is connected to the emitter electrode of transistor Q₁₇. The base electrode of transistor Q₃₁ is connected to the emitter electrode of transistor Q₁₉ through a resistor R₃₁ and to the emitter electrode of transistor Q₁₇ through a resistor R₃₂.

Next, the operation of the other portion 12' of the pre-driving circuit is described. Since most base currents of transistors Q₂₂, Q₂₃ and Q₃₂ flow through transistor Q₃₃, the collector current of transistor Q₃₂ is substantially constant and equal to the current value I_(D) of constant current source I₁₂. Therefore, the voltage across the base and emitter electrodes of transistor Q₃₂ is substantially constant. Detected currents I_(A) and I_(B), which are supplied from detecting circuit 14, flow through resistor R₃₃. When the voltage drop across resistor R₃₃ increases in response to the increase of the detected currents I_(A) and I_(B), the base potential of transistor Q₃₂ decreases in proportion to the increase of the voltage drop across resistor R₃₃. Accordingly, the base potential of each transistor Q₂₂ and Q₂₃ decreases and its collector current (i.e., the pre-driving current I_(C)) increases. On the contrary, in response to the decrease of the detected currents I_(A) and I_(B), the pre-driving current I_(C) decreases.

As a result, the pre-driving current I_(C) changes in response to demand and need not to be constant and large. Accordingly, this power amplifier has the same advantages as the embodiment shown in FIG. 1.

Restricting circuit 15 has the same function as restricting circuit 15 of the embodiment of FIG. 1. Further, the function of second restricting circuit 16 is as follows. When the voltage between the base and emitter electrodes of transistor Q₃₁ is larger than the predetermined value at the positive half cycle of the audio signal, transistor Q₃₁ conducts. Therefore, one portion of the pre-driving current I_(C) from transistor Q₂₂ flows through transistor Q₃₁. As a result, the excessive current is prevented from flowing through transistors Q₁₉ and Q₁₇. Resistors R₂₀, R₂₁, R₃₁ and R₃₂ have the following relationship so that transistor Q₃₁ can begin to conduct before the transistor Q₂₅ conducts:

    (Rb/Ra), >(Rd/Rc),

where Ra, Rb, Rc and Rd are the values of resistors R₂₀, R₂₁, R₃₁ and R₃₂, respectively. It is preferable that a circuit like second restricting circuit 16 is likewise employed for transistors Q₂₀ and Q₂₂ for the negative half cycle of the audio signal.

The power amplifiers shown in FIGS. 1 and 3 may be modified as mentioned below. Resistors R₂₁ and R₂₄ may be removed so that detecting circuit 14 operates more quickly. Further, the detecting currents I_(A) and I_(B), from which an AC component is removed, are supplied to the other portion 12' of the pre-driving circuit. Field-effect transistors also may be employed in the place of transistors Q₁₇, Q₁₈, Q₂₅, Q₂₆ and Q₃₁.

Although illustrative embodiments of the invention have been described in detail with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or sprirt of the invention. 

We claim:
 1. A power amplifier having an input terminal for receiving an audio signal comprising:a power amplifier circuit including a driving circuit and an output circuit having push-pull transistors, said driving circuit having an output for driving said output circuit; detecting circuit means coupled to said power amplifier circuit for generating a detected current signal which increases in response to an increase in the level of the output of said driving circuit and decreases in response to a decrease in the level of the output of said driving circuit; and pre-driving means coupled to said input terminal of said power amplifier and said power amplifier circuit for receiving the detected current signal of said detecting circuit means and supplying said driving circuit with a pre-driving current which increases in response to an increase in said detected current signal and decreases in response to a decrease in said detected current signal.
 2. The power amplifier of claim 1 comprising restricting circuit means for restricting the operation of said pre-driving circuit means so that the pre-driving current from said pre-driving circuit means does not increase over a predetermined value.
 3. The power amplifier of claims 1 or 2 comprising second restricting circuit means for restricting the output of said driving circuit so that the output of said driving circuit does not increase over a predetermined value. 